Bone grafts used in current orthopedic and neurosurgery procedures often serve two different functions. In certain surgical indications a graft is needed to act as a scaffold to aid the new bone in completely bridging a defect. In this type of graft, the material is typically porous to allow for bone ingrowth into the graft. Porous bone grafts act as a scaffold or trellis that allows regenerating bone to heal across a defect that it normally could not. Cancellous bone and porous ceramics have typically been used as bone graft scaffolds. Structural grafts, on the other hand, are bone graft materials whose main function is to mechanically support the site and add stability during the healing process. These materials have little to no porosity but have the strength necessary for stabilization. Cortical bone struts, rings, and wedges are common examples of structural grafts.
The source of the bone-derived grafts is either from the patient's own cancellous or cortical bone (autograft) or from tissue donors (allograft). Although these bone grafts have been successfully used over the years, they are not devoid of certain disadvantages. Removing healthy bone from the patient and placing at another site often results in complications of pain and infection at the donor site. Using a tissue donor can result in variable resorption characteristics and unpredictable structural integrity. In an attempt to avoid the problems of autograft and allograft, synthetics have become a popular choice for orthopedic surgeons. Resorbable polymers, ceramics, and composites have been shown to be effective substitutes for bone derived grafts without any of the autograft or allograft related complications.
From a scaffold standpoint, current porous implants are used as graft materials primarily for bone and cartilage repair. These scaffolds are characterized by a high percent porosity to allow for bone and/or cartilage in-growth. A variety of pore forming techniques used to create three-dimensional porous scaffolds are known. These techniques, however, result in structures that can be easily crushed or deformed due to their low strength. In orthopedic grafting procedures, surgeons often use force to impact the graft material into the site, which may crush or fracture the graft material. Thus, if the graft material has low mechanical properties, the porosity can be significantly reduced or completely eliminated if the graft is crushed or fractured during or after the surgical procedure. With little to no porosity remaining, the graft can no longer function as a scaffold for tissue in-growth. Therefore, it would be advantageous to create a tissue scaffold with higher strength to prevent crushing during the implantation procedure.
For structural grafting applications, current implants are typically composed of non-degradable materials such as titanium, poly(ether ether ketone) [PEEK], and polyethylene. Although these high strength materials possess the required mechanical properties, their function is only required during the healing process. Once the site has fully healed, the implant serves no purpose and can be a source of long term complications such as loosening or failure. This has driven the search for resorbable implants that perform their mechanical function and are then resorbed by the body. Polymers such as poly (lactic acid) (PLA) and poly (lactide-co-glycolide) (PLG) have been widely used to create resorbable implants for structural applications such fracture fixation, reconstruction, and spinal fusion. However, one issue with typical resorbable polymer implants is that they maintain their volume until the very end of resorption. This can lead to a void in the tissue that may or may not eventually fill in. It would be advantageous to have a material that would allow tissue in-growth into the implant prior to complete resorption of the device.